Method of producing a coated anode

ABSTRACT

An electrode useful for electrowinning and other processes including evolution of oxygen or chlorine or for plating is formed of titanium particles, compacted cold, and coated and cemented with a first layer of manganese dioxide thermally deposited from Mn (NO3)2 on the grains to form a coating of a combination of manganese dioxide and titanium oxide having a rutile crystal structure, and a further outer layer of manganese dioxide which is electrodeposited, the two coatings not being limited to the surface of the electrode but extending to all exposed surfaces of the grains including those which are walls of channels between the grains. The compression of the titanium powder is to a density between 30 and 70 percent of the density of solid metal. Solid metal such as metal mesh may be included in the titanium powder prior to compacting, such as expanded titanium metal, to assist in strengthening the form. In an alternate embodiment the metal substrate is lead.

United States Patent Feige, Jr.

[45] Dec. 17, 1974 METHOD OF PRODUCING A COATED ANODE [76] Inventor:Norman G. Feige, Jr., Ridgefield Ave., R.F.D. No. 1, South Salem, NY.10590 [22] Filed: July 18, 1973 [21] Appl. No.: 380,325

[52] US. Cl. 204/38 R, 204/96, 204/290 F, 204/290 R, 204/280 [51] Int.Cl. B011: 3/06, C23b 11/00, COlb 7/06 [58] Field of Search 204/38 R, 290R, 290 F,

[56] References Cited UNITED STATES PATENTS 2,867,570 111959 Dufour etal. 204/83 3,278,410 lO/l966 Nelson 204/285 3,535,217 10/1970 Arnano eta1 204/96 3,634,216 lll9 72 Gibson et al 204/83 3,663,280 5/1972 Lee117/217 3,689,384 9/1972 Barbato et al. 204/99 3,711,397 l/l973Martinsons 204/290 F Primary Examiner-F. C. Edmundson Attorney, Agent,or FirmWheeler, Morsell, House & Fuller I [57] ABSTRACT An electrodeuseful for electrowinning and other processes including evolution ofoxygen or chlorine or for plating is formed of titanium particles,compacted cold, and coated and cemented with a first layer of manganesedioxide thermally deposited from Mn (N0 on the grains to form a coatingof a combination of manganese dioxide and titanium oxide having a rutilecrystal structure, and a further outer layer of manganese dioxide whichis electrodeposited, the two coatings not being limited to the surfaceof the electrode but extending to all exposed surfaces of the grainsincluding those which are walls of channels betweenthe grains. Thecompression of the titanium powder is to a density between 30 and 70percent of the density of solid metal. Solid metal such as metal meshmay be included in the titanium powder prior to compacting, such asexpanded titanium metal, to assist in strengthening the form. In analternate embodiment the metal substrate is lead.

14 Claims, 4 Drawing Figures METHOD OF PRODUCING A COATED ANODEBACKGROUND OF THE INVENTION It has long been known that titanium metalhas superior properties for use as an electrode in cells, baths orsolutions which would corrode and be contaminated by metal fromelectrodes of other compositions. Nevertheless there are difficulties inthe use of a titanium electrode as well. Solid titanium leads to arequirement for high voltage, and gives poor current efficiency. Fox US.Pat. Nos. 2,631,115 and 2,608,531 discuss some of the difficulties inconnection with specific systems, for instance the passive surfacecoating on the titanium, and indicate that at least in the production ofoxides such as manganese dioxide for use in batteries as a depolarizer,the use of a porous titanium anode made of chips is of assistance. Undercertain special conditions described by Fox, the plating of MnO or in abattery, the use of a specific form of titanium chips of about 35 meshin one case and the use of specific voltage relationships duringelectrolytic formation of the surface of the electrode prior to use inthe other patent, improved results are observed using porous titaniumanodes. Fox discloses coating the surfaces of the porous titanium masswith graphite, gold or iron, or with any good conductor which is inertto the elctrolyte in which the electrode is to be used. Among otherthings Fox discloses that his electrodes are useful for the preparationof electrolytic manganese dioxide. His described electrode does notfunction as an oxygen evolution anode, but passivates under thoseconditions.

In industry it is most important to select a suitable anode that doesnot contaminate the electrolyte or contaminate the cathode deposit, thathas a long life, and has a low oxygen overvoltage during electrolysis.Platinum is an excellent known anode material which satisfies the abovementioned characteristics.

Recently platinum and other precious metals have been applied to atitanium substrate to retain their attractive electrical characteristicsand further reduce manufacturing cost. However such anodes are expensiveand are not suitable for some industrial uses. Thus carbon and leadalloy electrodes have generally been used. The carbon anode, however,has the disadvantage that it greatly contaminates the electrolyte, wearsfast, and has high electrical resistance which results in an increase incell voltage. It may also be degraded to CO during oxygen evolution. Thedisadvantage of the lead alloy anode is that PbO changes to Pb O whichis poorly conductive. 0 gets below this layer and flakes off the film.These particles become trapped in the deposited copper at the cathode,degrading it.

In order to overcome these disadvantages it has recently been proposedto plate the surface of a titanium substrate with platinum and toelectrodeposit either lead dioxide or manganese dioxide on the platedsurface. Such anodes have the disadvantage of comparatively high oxygenovervoltage. In addition both coatings have high internal stress whenelectrolytically deposited and are liable to be detached from thesurface during commercial usage, both contaminating the electrolyte and,in the case of lead, being deposited on the cathode to reduce its value.Thus current density with such anodes is very limited.

To improve the high oxygen overvoltage it has also been proposed tocompact and sinter titanium chips to increase the apparent surface area.Such an anode has somewhat improved characteristics but does notproperly receive and retain the electrodeposited manganese dioxidecoating.

This invention is based on recognition that deposited manganese dioxideis both insoluble and electrically conductive, and cannot readily bedeposited as a reduced product on the cathode. Experiments have shownthat my electrode has a low oxygen overvoltage during electrolysis,economizes on electric power necessary for electrolysis, reduces theloss of manganese from the electrode to the bath to a very low figure,and is believed to be the optimum electrode for electrolytic winning ofcopper, zinc and nickel in sulfate electrolytes. It is also useful forevolving oxygen in sulfate systems and chlorine in chloride systems.

SUMMARY OF THE INVENTION The preferred electrode of my invention is madeof titanium powder cold compacted to the shape of an electrode, thecompaction being sufficient to produce a density in the powder between30 and percent of the density of the solid metal of which the powder iscomposed. A layer of manganese dioxide is produced on the surface of thegrains throughout the mass of the electrode by thermal decomposition ofMn (NO That layer is modified by ion exchange with the titanium dioxidesurface layer and probably with the titanium metal of the powder grainsto contain titanium atoms as well as manganese atoms. These do notsignificantly modify the crystal structure which is essentially that ofrutile. This mixed structure is formed while the material of the coatingis in the intermediate manganeous-manganite form during thedecomposition from manganeous nitrate to manganese dioxide.

A final coating of manganese dioxide is electrodeposited over the hybridcoating.

Preferably the structure is strengthened by including solid metal in theelectrode. A preferred form is an expanded metal lattice of titaniumalthough other shapes may be used. The preferred size range of thetitanium powder is mesh to -325 mesh. The preferred thickness of theouter coating of mananese dioxide is 100 microns. The density referredto is calculated on the weight of the powder alone. The upper limit isthe loss of connected pores between the particles.

The preparation of an electrode in this manner produces a strongerelectrode without the necessity of sintering the powder because theinitial thermally deposited layer is extremely effective in cementingthe grains of the powder. It has been found that even loose titaniumpowder may be cemented by this method to produce a coherent shape.Powder which has been cold compacted to between 30 and 70 percent ofmetallic density and then coated as described produces an extremelydurable electrode suitable for use as the anode in an electrowinningprocess. The anode evolves O in sulfates and Cl in chlorides, andaccordingly may also be used for the evoluation of chlorine as well.

It has been observed that manganese dioxide coating of prior artelectrodes used in electrowinning is lost during periods of shutdown, atwhich time it goes into solution and becomes a part of the bath,degrading the electrode. The loss is largely in the form of conversionto manganese cations and permanganate. With the anode of my invention,the ions and permanganate are largely limited to the pores or channelsbetween the grains of titanium powder and a very high percentage isredeposited as manganese dioxide upon application of current for theelectrowinning process, in the same manner that the outer layer wasoriginally deposited. Thus degradation of the anode and pollution of thebath are both avoided.

Finally the electrical and operating characteristics of my electrodecompare favorably with prior art electrodes.

The chief presently known use of my electrode is as an anode forelectrolysis. So used, an advantage of the anode is low oxygenovervoltage during electrolysis, thus economizing the electrical powdernecessary.

A similar electrode may be made with lead particles, at lower cost. Asin the case of the titanium electrode a first coating of MnO isthermally deposited and forms a hybrid with the lead oxide surface layernormally present. A further layer of MnO is electrodeposited. Areinforcement of solid metal, which may be titanium expanded metal mesh,may be used.

DRAWINGS FIG. 1 is a perspective view of a rectangular electrode madeaccording to my invention with portions broken away to show interiorstructure.

FIG. 2 is an enlarged cross-sectional view on line 2-2 of FIG. 1.

FIG. 3 is a graph of cell voltage against current density for an anodeaccording to my invention compared with a lead anode and a solidtitanium anode coated with MnO FIG. 4 is an idealized cross-section ofseveral grains showing the two layers and the pores highly magnified.

DESCRlPTlON OF THE PREFERRED EMBODIMENTS Although the disclosure hereinis detailed in order to enable those skilled in the art to practice theinvention, the embodiments disclosed merely exemplify the inventionwhich may take other forms. The scope of the invention is defined in theclaims.

FIG. 1 is a broken away perspective view showing a portion of anelectrode 10 formed according to my invention. An expanded metal mesh 12of titanium metal has been placed with l mesh titanium powder in a moldand compacted in a press until the powder has a density of 30 to 70percent of theoretical density of the metal. The precise shape is not apart of my invention and may conform to specifications for a particularuse, so the outline is not shown. FIG. 2 is a cross-sectional view. Asshown in FIG. 4, which is a highly magnified and idealizedcross-sectional view through several particles of titanium, each grain20 is coated with a first layer 22 and a second layer 24, leavinginterconnected pores 26 between the grains.

The first layer 22 is basically MnO which has been deposited by thermalbreakdown of Mn (N0 and which has exchanged ions with the metal andsurface layer of titanium oxide normally present on the metal and notseparately shown. Thus layer 22 has a modified rutile character. It isstrongly adherent and cements the grains 20 and the mesh 12 together.

The second layer 24 is electrodeposited MnO It is not known tointerchange ions with the first layer and is believed therefore to beidentical with commercially electrodeposited manganese dioxide.

The pores 26 are the spaces between the coated grains. As more fullydescribed elsewhere, they are highly interconnected. This not onlycreates high available surface area (as opposed to surface in a closedcavity not connected through pores 26 with the exterior of the electrode10) thus improving the apparent current density, it also means that asubstantial part of the first and second layers 22-24 are in the pores.During inactivity of the immersed electrode Mn0 can break down tomanganese cations and permanganate. In my electrode these are largelytrapped in pores 26. Upon reapplication of current to the anode, Mn0 isre-formed on the second layer with very little loss.

The method of my invention thus comprises the basic steps of compactingmetal particles to a density sufficient to make a coherent final productbut not so high as to destroy the high interconnection between thepores, thermally breaking down Mn (N0 at a temperature between about 120C. and 475 C. to form a first layer on the particles or grains, anddepositing a second layer of Mn0 over the first layer, to form anelectrode. The metal may preferably be titanium but may also be lead.

The density of the compacted titanium powder used in my electrode hasboth a lower and upper limits for satisfactory performance. The freelypoured titanium powder of a typical sieve analysis as follows:

TABLE 1 Ti Powder Analysis Typical Sieve Analysis Size All powder lessthan -l00 mesh.

l00, +l50 mesh 25 percent l50, +200 mesh 21 percent -200, +270 mesh 26percent 270, +325 mesh l7 percent --325 mesh ll percent Chemistry Oxygen0.30 max. Nitrogen 0.04 max. Carbon 0.04 max. Chlorine 0.20 max. Iron0.50 max. Magnesium 0.30 max. Total others 0.12 max. Titanium 98.50 min.

has a theoretical density of 25 percent, that is, its weight is 25percent of the weight of an equal volume of solid titanium metal havingthe same anlaysis. Using great care it is possible to compact suchpowder to only 30 percent density in the shape of an anode and cementthe grains of the powder together by thermal decomposition of Mn (N09 asdescribed elsewhere in this specification. Thus 30 percent density isbelieved to be a practical lower limit of density. With greatercompacting force the density of the powder may beincreased to aboutpercent while retaining highly interconnected passageways between thegrains of metal of sufficient size for the application of the coatingsof my electrode. Above 70 percent theoretical density the pores loseinterconnection and surface area is lost to the extent that followingthe process described in this specification does not result indeposition of sufficient MnO either in the layer produced by thermaldecomposition or in the electrodeposited layer. Above 70 percent densitythe oxygen overvoltage rises unduly as shown in table 2 below:

TABLE No. 2

The Relationship Between Theoretical Density And Anode Performance Anode%Den. Thickness %MnO Another method of defining the upper limit is byblowing air through the compacted mass. Substantial resistance to thepassage of air indicates that few passages are interconnected, showingthat compaction is too great. The upper limit may vary with particlesize and shape but includes only a degree of compaction which leaves thepores highly interconnected.

In my electrode, the grains of titanium powder are cemented together bythe first coating of M1102 which is produced by thermal decomposition ofMn (NO between 120 C. and 475 C. During the thermal decomposition thereis an extensive reaction with the surface titanium oxide layer which isnormally present on titanium metal by ion exchange, during the phasewhen the coating is black liquid manganeous-manganite. The result is anadherent coating bonding the grains of titanium powder with a highlyconductive oxide film. Subsequently a further layer of MnO iselectrolytically deposited, resulting in an electrode having extensivelyinterconnected passages between the grains, the surface of each grainboth on the surface of the electrode and in the passages being coatedwith a first layer of modified rutile containing titanium, and then withMnO Electrodeposited manganese dioxide is brittle, and has largeinternal stresses. It is readily detached from a substrate whendeposited to an appropriate thickness for electrode use, making itdifficult to form an effective and long lived electrode from suchmanganese dioxide alone. My invention permits the application of heavydeposits in excess of 100 microns thick of manganese dioxide within thepores between the grains of titanium powder, thus taking advantage ofthe large internal stresses of the coating to improve its adherencerather than to cause failure as in the case of a flat sheet electrode.

Finally, by depositing much of the coating internally it is protectedboth from mechanical dislodgement and from loss into the solution whenthe electrode is inactive.

Although under anodic potential as applied during electrolysis,manganese dioxide is insoluble, under an open circuit permanganate isobserved in solution. As

the potential is again applied to the anode the permanganate isredeposited on the anode as manganese dioxide. By coating the poroussubstrate internally the dissolution of manganese dioxide is reduced andthe concentration of permanganate is sufficiently high in the pores forredeposition to reduce significantly the loss of manganese from theanode and the pollution of the bath.

EXAMPLE 1 An example showing the comparison between my electrode and asimilar electrode made with solid titanium plate is as follows:

Two sheets of expanded titanium mesh, each 1.5 millimeters thick, wereplaced in a mold 500 centimeters by 7.50 centimeters by 3 millimetersthick with 82 grams of titanium powder having the sieve analysis andchemical analysis shown in Table 1. These were subjected to a pressureof 5,400 kilograms, resulting in a composite structure in which thepowder component had a theoretical density of 52 percent of the densityof solid titanium. The electrode was impregnated with an aqueoussolution of Mn (N09 and was baked at 176 Centigrade. The electrodedeveloped a gray adherent coating. The electrode was then placed in abath of MnSO, and H at Centigrade and current was applied toelectrolytically deposit a coating of manganese dioxide followingaccepted techniques. It was found that during this step current densitycould be varied from 0.1 ampere to 6.7 amperes per square foot, whichwas the observed oxygen evolution value for the anode. A black layer ofMnO developed over the first layer.

In the same manner a titanium plate 500 centimeters by 750 centimetersby 3 millimeters thick having a sandblasted surface was coated with twolayers as described above.

The two electrodes thus produced are compared in FIG. 3 along with anelectrode of solid lead. The bath was a copper plating bath containingCuSO, and H 80, at 60 Centigrade and a gap of '15 inch. The anode madefrom titanium plate was less durable and required higher cell voltage,despite the use of both thermal and electrolytic deposition of MnO inaccordance with a portion of my invention. Line 34 is the curve for theporous anode with two layers made according to my invention. Line 32 isthe curve for lead. Line 30 is the curve for the solid titanium anodewith two MnO layers.

A porous anode as described in this example was used for electrolyticwinning of copper, zinc and nickel in the respective commercialsulfate-sulfuric acid electrolytes. The anode performed satisfactorilyand exhibited a substantial improvement in cell voltage in the system ascompared to lead anodes in each electrolyte.

The porous electrode described has also been tested as an anode forevolution of oxygen, and as an anode for evolution of chlorine, bothwith good efficiencies and service life. The anode is useful forchlorination of water, for instance.

EXAMPLE 2 The effect of changes in the density of the powder componentof my electrode may be seen in table 2 below. Anodes prepared accordingto the first paragraph of Example 1, with the exception that thecompaction and amount of MnO varied, are compared.

TABLE NO. 2

The Relationship Between Theoretical Density And Anode Performance AnodeDen. Thickness M110 Back EMF Cell Voltage at 24 amps/ft at 120 amps/ft72% 1.0cm 2% 1.15 volts 1.55 1.95 63% 1.1cm 3% 1.20 volts 1.54 1.90 49%1.0crn 1.06 volts 1.43 1.87 46% 1.6cm 6% 1.20 volts 1.48 1.84 38% 2.1cm14% 1.11 volts 1.35 1.69 37% 1.2cm 17% 1.08 volts 1.30 1.71

EXAMPLE 3 changes atoms with the titanium and titanium oxide in a mannerlike that of Example 1, lead electrodes were prepared using lead shotparticles and titanium expanded metal mesh but using lower pressure. Inrespective trials lead paraticles ranging from no. 6 shot (0.110 in.) tono. 11 shot (0.065 in.) were used. After the two layers of MnO aredeposited as in Example 1, the electrode was tested as an anode in acopper sulphate and sulfuric acid conventional electrolyte. The graph ofcell voltage vs. amperes/sq. ft. for this electrode was intermediatebetween such a graph for a solid lead anode and that for the anode ofExample 1 for the particle sizes tested.

It will be understood that the invention is not limited to the examplesdescribed and that many modifications may be introduced therein. Thescope of the invention is intended to be limited only by the scope ofthe appended claims.

1 claim:

1. A method of manufacturing an electrode comprising the steps of:

first, cold compacting metal particles chosen from the group consistingof titanium and lead to form an electrode having an extensivelyinterconnected pore system between the particles;

second, depositing a first layer of manganese dioxide on the exteriorsurface of the compact and within the pores between particles by thermaldecomposition of a solution of Mn (N09 and third, electrodepositing alayer of manganese dioxide on said first layer.

2. The method of claim 1 in which the particles are titanium.

3. The method of claim 1 in which the particles are lead.

4. The method of claim 2 in which the particles comprise powderpredominantly in the size range of +100 mesh to -325 mesh.

5. The method of claim 1 in which the particles are compacted along withat least one solid titanium metal reinforcement to increasev themechanical strength of the electrode.

6. The method of claim 2 in which during the second Step, themiaslslansansss 193 29..s9 @q normally present on the surface of theparticles to form a modified rutile crystal structure which iselectrically conductive.

7. The method of claim 1 in which the electrodeposited manganese dioxideis approximately 100 microns thick.

8. The method of claim 1 wherein the density of the compacted particlesis between 30 and percent of the density of the solid metal of which theparticles are composed.

9. A method of forming an electrode comprising the steps of:

first, forming the body of the electrode from particles of titanium;second, thermally decomposing manganeous nitrate in contact with theelectrode to form a layer of the intermediate productmanganeous-manganite;

third, causing ion exchange between the body of the electrode and thecoating to replace manganese ions with titanium ions;

fourth. causing the product to coat the surfaces of the particles toform an adherent coating of a highly conductive film ofmanganese-titanium oxide;

and fifth, thereafter electrodepositing a layer of manganese dioxide onthe surfaces of the coated particles.

10. The method of claim 9 in which the second step cements the grains orparticles together in the body of the electrode thereby forming amechanically sound electrode for use as an anode.

11. The method of claim 9 in which the powder is in the size range ofmesh to 325 mesh.

12. The method of claim 9 in which the powder is compacted along with atleast one solid titanium metal reinforcement to increase the mechanicalstrength of the electrode.

13. The method of claim 9 in which during step 2 the manganese dioxidecoating is reacted with the titanium oxide normally present on thesurface of the particles to form a modified rutile crystal structurewhich is electrically conductive.

14. The method of claim 9 in which the electrodeposited manganesedioxide is approximately 100 microns thick.

1. A METHOD OF MANUFACTURING AN ELECTRODE COMPRISING THE STEPS OF:FIRST, COLD COMPACTING METAL PARTICLES CHOSEN FROM THE GROUP CONSISTINGOF TITANIUM AND LEAD TO FORM AN ELECTRODE HAVING AN EXTENSIVELYINTERCONNECTED PORE SYSTEM BETWEEN THE PARTICLES; SECOND, DEPOSITING AFIRST LAYER OF MANGANESE DIOXIDE ON THE EXTERIOR SURFACE OF THE COMPACTAND WITHIN THE PORES BETWEEN PARTICLES BY THERMAL DECOMPOSITION OF ASOLUTION OF MN (NO3)2; AND THIRD, ELECTRODEPOSITING A LAYER OF MANGANESEDIOXIDE ON SAID FIRST LAYER.
 2. The method of claim 1 in which theparticles are titanium.
 3. The method of claim 1 in which the particlesare lead.
 4. The method of claim 2 in which the particles comprisepowder predominantly in the size range of +100 mesh to -325 mesh.
 5. Themethod of claim 1 in which the particles are compacted along with aTleast one solid titanium metal reinforcement to increase the mechanicalstrength of the electrode.
 6. The method of claim 2 in which during thesecond step the forming manganese dioxide coating interchanges atomswith the titanium and titanium oxide normally present on the surface ofthe particles to form a modified rutile crystal structure which iselectrically conductive.
 7. The method of claim 1 in which theelectrodeposited manganese dioxide is approximately 100 microns thick.8. The method of claim 1 wherein the density of the compacted particlesis between 30 and 70 percent of the density of the solid metal of whichthe particles are composed.
 9. A method of forming an electrodecomprising the steps of: first, forming the body of the electrode fromparticles of titanium; second, thermally decomposing manganeous nitratein contact with the electrode to form a layer of the intermediateproduct manganeous-manganite; third, causing ion exchange between thebody of the electrode and the coating to replace manganese ions withtitanium ions; fourth, causing the product to coat the surfaces of theparticles to form an adherent coating of a highly conductive film ofmanganese-titanium oxide; and fifth, thereafter electrodepositing alayer of manganese dioxide on the surfaces of the coated particles. 10.The method of claim 9 in which the second step cements the grains orparticles together in the body of the electrode thereby forming amechanically sound electrode for use as an anode.
 11. The method ofclaim 9 in which the powder is in the size range of +100 mesh to -325mesh.
 12. The method of claim 9 in which the powder is compacted alongwith at least one solid titanium metal reinforcement to increase themechanical strength of the electrode.
 13. The method of claim 9 in whichduring step 2 the manganese dioxide coating is reacted with the titaniumoxide normally present on the surface of the particles to form amodified rutile crystal structure which is electrically conductive. 14.The method of claim 9 in which the electrodeposited manganese dioxide isapproximately 100 microns thick.